This invention relates to imaging optical systems and more particularly to imaging optical systems utilizing a staring two-dimensional matrix array of detectors disposed in a focal plane for image detection and processing.
As is known in the art, optical systems have a wide variety of applications including use in infrared missile seekers. One type of an infrared missile seeker includes a gimballed focusing system for focusing infrared energy from an external scene which may include a target, into an image on a focal plane within the seeker. At the focal plane, an array of detectors are disposed for detecting infrared energy impinging thereon, the signals produced by the array of detectors indicating the presence and characteristics including location on the field-of-view of a target as well as other background objects.
Various methods of processing infrared energy to obtain an indication of a target include the techniques disclosed in U.S. Pat. No. 3,872,308, issued Mar. 18, 1975, inventors James E. Hopson and Gordon G. MacKenzie, U.S. Pat. No. 4,339,959, issued Jul. 20, 1982, inventors Benjamin Klaus, Jr. and Gordon C. MacKenzie and U.S. Pat. No. 5,072,890, issued Dec. 17, 1991, inventors Benjamin Klaus, Jr., Gordon C. MacKenzie and Richard A. Beckerleg, U.S. Pat. Nos. 3,872,308, 4,339,959 and 5,072,890 being assigned to the same assignee as the present invention and incorporated herein by reference.
An infrared seeker with a detector array disposed at a focal plane forms an image (i.e. picture) of a scene within the field of view of the infrared seeker. The image is formed from a plurality of pixels (i.e. picture elements) with each pixel corresponding to each detector element of the detector array. For example, a 128.times.128 element detector array would provide an image with 128.times.128 pixels and a 256.times.256 element detector array would provide an image with 256.times.256 pixels. For images that are to be further processed by a computer, the image would most likely be represented by an array of digital words, with each word indicative of the intensity of the infrared energy from a small area within the instantaneous field-of-view corresponding to one detector element of the detector array. Thus, the outputs from the detector array are digitized by an analog to digital converter and an image is provided as a two dimensional array of pixels, each pixel represented by a digital word. The image is then processed by a computer or processor as is known.
One problem with an infrared seeker using a focal plane array is that each detector in the array of detectors has varying characteristics including a DC offset voltage and gain/responsivity due to physical and electrical irregularities. These varying characteristics result in noise like image irregularities (commonly referred to as fixed pattern noise) which are largely uncorrelated in space (i.e., from element to element) and relatively fixed in time (i.e., from frame to frame). To minimize these varying characteristics, improved production processes are used to reduce the physical and electrical irregularities. Although adequate for some instances, additional compensation is generally required to further reduce the fixed pattern noise, particularly in imaging infrared applications.